The Influence of Chloride Concentration

Since 1930, it has generally been accepted that among all anions, chloride ions have the highest power of penetration into natural oxide film, because they are small and very mobile. Their dimension is close to that of oxygen [12]. Chlorides (Cl ), as well as fluorides (F ), bromides (Br ) and iodides (I ) belong to the anions that activate corrosion of aluminium in water, while sulphates (SO ), nitrates (NOJ) and phosphates (PO ) hardly activate it or do not activate it at all [13]. 

Chlorides may substitute oxygen atoms in the alumina network. This leads to a decrease in the film s resistivity, which facilitates the release of aluminium atoms that diffuse into the water [13]. The aggressivity of a medium with respect to metals in general and aluminium in particular depends to a large extent on its chloride and sulphate concentration. 

At a given chloride concentration (Cl ), there is no perceptible difference between the action of a solution of potassium chloride (KCl), sodium chloride (NaCl) or ammonium chloride (NH4CI). 

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